The invention relates generally to power plants and power generation equipment. More particularly, the invention relates to techniques for monitoring heat transfer devices of various types in power plants or similar installations.
Power plants and power generation equipment play a significant role in the current economy. The demand for electrical power has never been greater, and it has been estimated that in the coming years the demand will continue to increase. Power plants are typically classified based on the process of electrical power generation. Examples of types of power plants include thermal power plants, hydroelectric power plants, nuclear power plants, geothermal power plants, wind farms, and solar farms. Each generating modality operates with a specific and different input and produces an electrical power output. In most power plants, heat transfer devices such as furnace firewalls, air preheaters, superheaters, reheaters, economizers, and so forth are used to transfer heat from one medium to another medium in order to maintain a continuous generation of electrical power.
When the heat transfer devices function, there are a number of processes that take place. Typically, these processes may produce undesired by-products that can subsequently hinder the functioning of the processes. As a result, the efficiency of the power plant decreases with time and overall cost of operation of the power plant increases. Boilers, for example, form an integral part of any power plant that uses combustion to generate steam to run turbines. Boilers generate steam by heating a liquid medium, typically by combustion of a fuel. Common types of fuel used to fire boilers include coal and fuel oil. Due to chemical by products released during combustion and incomplete combustion, the boilers commonly suffer from fouling and slagging.
Fouling is the accumulation of ash and by products of incomplete combustion as a layer on the inner surface of a boiler. A layer of fouling substance typically has low thermal conductivity that reduces the amount of heat transferred in heat exchanging components. In the case of a boiler, this causes an overall increase in temperature inside the boiler for a given amount of steam production, a decrease in functional efficiency of the boiler, and an increase in pressure drop within the boiler due to reduced cross-sectional area. Slagging is a process that is quite similar to fouling. Slagging refers to deposition of solid or molten layers on the outside surface of the boiler tubes. Such layers are formed by a process commonly known as sintering. Sintering is the bonding of adjacent surfaces of particles into a hard deposit. Sintering subsequently strengthens the mass, causing an increase in tenacity of the deposit on the surface. Both fouling and slagging are self-propelling processes, meaning that once an initial layer of a material forms on a surface, the layer causes more of the material to be deposited.
Another of the components used for heat transfer in power plants and similar installations is an air preheater. The air preheater typically transfers heat from re-circulating flue gases to air. The outgoing flue gases from a combustion process are used to pre-heat air that could be used in other processes. The pre-heating of air reduces the amount of energy spent in raising the temperature of an otherwise cold air to a desired temperature level. Because these flue gases contain organic and inorganic chemicals, the flue gases react with metal tubes disposed within the air preheater and cause corrosion and fouling.
Due to effects as described above, performance of these components drops significantly over time and ultimately causes a breakdown or an expensive forced outage. Addition of chemicals to the fuel and treating the device surface is a common way of reducing the effects of fouling and slagging. The addition of chemicals can be an expensive process and also is time consuming. The existing systems that monitor boilers are typically what can be termed as reactive warning systems. This means that the systems would raise an alarm or warning only when a problem affects the boilers or other heat transfer devices. By the time the alarm is raised, the effects of fouling and slagging typically have progressed to a severe stage that leaves system operators with little option other than to shutdown the boiler for cleaning and/or treating the heat transfer surfaces to bring the boiler performance to acceptable limits. Also, such reactive warning systems generally are incapable of predicting when a failure of the boiler might occur. In many cases, evaluation relies instead on the operator experience and knowledge of boiler performance to predict an impending failure.
There is, therefore, a need for an active warning system that consistently and continuously monitors the performance of heat transfer devices and alerts a system operator of an impending problem, not when the problem has occurred but when the problem is actually at its initiation phase, and is also able to predict more accurately when a device failure might occur. With such an early warning, maintenance of heat transfer devices would be easier and would require lower levels of chemicals to treat the devices and fuel, leading to a longer operating life and a significant reduction in maintenance costs.